1. Field of the Invention
The present invention relates to a motor-operated power steering apparatus for a vehicle having steerable front and rear wheels, and more particularly to a rack-and-pinion motor-operated power steering apparatus having an electric motor for generating assistive steering power and transmitting such assistive steering power to the steerable front and rear wheels of a vehicle to turn the wheels.
2. Description of the Relevant Art
There are known motor-operated power steering apparatus having an electric motor for generating assistive steering power in a steering force transmitting system for front dirigible wheels. The assistive steering power produced by the motor is transmitted, together with manual steering forces applied to a steering wheel, to the front wheels thereby to lessen the steering efforts of the driver of a motor vehicle equipped with such a motor-operated power steering apparatus.
For example, U.S. Pat. No. 4,415,054 discloses a rack-and-pinion motor-operated power steering apparatus comprising a rack shaft axially movable supported in a rack casing, a pinion shaft having an upper end operatively coupled to a steering wheel and a lower end having a pinion gear meshing with rack teeth on one side of the rack shaft, and an electric motor disposed coaxially with the rack shaft for generating assistive steering power. The rack shaft has a helical groove defined on a central portion thereof and covered by the electric motor which is disposed in the rack casing. The electric motor comprises a number of magnets disposed on the inner peripheral surface of the rack casing in surrounding relation to the helical groove, a tubular armature surrounded by the magnets and disposed around the helical groove coaxially with the rack shaft, and a tubular ball nut fixed to the inner peripheral surface of the tubular armature and disposed around the helical groove coaxially with the rack shaft.
The electric motor is driven under the control of a signal representative of steering forces detected by a sensor associated with a torsion bar which interconnects the steering wheel and the pinion shaft. Rotative power generated by the motor is converted to axial movement of the rack shaft by means of a number of balls interposed between the ball nut member and the helical groove on the rack shaft.
The assistive steering power produced by the motor is transmitted, together with manual steering forces imposed on the steering wheel by the driver, to the front wheels, so that steering efforts can be reduced.
Recent years have seen the development of vehicles with steerable front and rear wheels, having a rear wheel steering mechanism for steering the rear wheels upon turning of the front wheels. Where the rack-and-pinion motor-operated power steering apparatus, as described above, is incorporated in such a vehicle with steerable front and rear wheels for the purpose of giving assistive steering power in turning the rear wheels as well as the front wheels, the motor-operated power steering apparatus must be mechanically coupled to the rear wheel steering mechanism through a connecting shaft and other members whereby the assistive steering power produced by the motor will be transmitted to the rear wheels to turn the rear wheels in response to turning of the front wheels.
The motor is positioned centrally on the rack shaft, and the pinion shaft coupled to the steering wheel is positioned on one side of the rack shaft, with the helical groove coupled to the motor and the rack teeth coupled to the pinion gear on the lower end of the pinion shaft being on the rack shaft substantially across the entire length thereof. Therefore, the design freedom for providing the rack shaft with a power output area for extracting assistive steering power for the rear wheels is substantially limited. Since such a power output area cannot be located in at least the central portion of the rack shaft, the connecting shaft cannot be positioned longitudinally through the central zone of the vehicle body which provides a dead space in the vehicle body, and hence good space utility cannot be achieved.
The power output area has to be positioned on one of the opposite ends of the rack shaft, or an extension thereof, or by dividing the rack shaft into two shaft members, in order that the motor-operated power steering apparatus can be connected to the rear wheel steering mechanism. However, such an arrangement results in an increased overall mechanism size, imposing limitations on the designing of the geometrical structure of front wheel steering linkages.